ST. STEPHEN S GIRLS COLLEGE Mid Year Examination PHYSICS Time Allowed: 1 hour 30 minutes NAME: F.3 ( ) MARKS:

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1 F.3 Physics Mid Year Examination page 1 Form students ST. STEPHEN S GIRLS COLLEGE Mid Year Examination PHYSICS Time Allowed: 1 hour 30 minutes YRKwong, WYYau NAME: F.3 ( ) MARKS: Please read the following instructions very carefully. 1. Answer ALL questions in the spaces provided. 2. This paper consists of TWO sections. Section A carries 70 marks, and Section B (multiple-choice questions) carries 30 marks. 3. Answer Section B in the spaces provided on the last page of this question paper. SECTION A (70 marks) 1. You are asked to make a liquid-in-glass thermometer. Table 1(a) shows some properties of two liquids. Table 1(a) Liquid Specific heat capacity/ J kg -1 o C -1 Expansivity/ o C -1 A B (a) Which liquid, A or B, will you choose for your thermometer? Explain your choice with TWO reasons. A ( ) It has lower specific heat capacity thus, it absorbs/releases less energy for the same temperature change and it won t affect the environment much during measurement. ( ) It has larger expansivity thus it expands/contracts more for the temperature change and will give a more accurate reading. (1 M) (b) Which of the two designs shown in figure 1(b) will you use? Assume the same amount of liquid is provided in the two designs. Explain your choice. Design I (1 M) It has a narrower tube thus the change in column will be larger for the same dt (1 M) The % error in reading the length will be smaller (1 M) Design I Design II Figure 1(b) (c) Your classmate made another liquid-in-glass thermometer and she found that the lengths of the liquid column at 10 0 C and 60 0 C are 6.4 cm and 19.1 cm respectively. (i) When the length of the liquid column is 12 cm, find the measured temperature. (12-6.4)/( ) = (T-10)/(60-10) (1 M) T = C (1 M) (ii) State the assumption that has been made in the calculations in (i). (1 mark) The length varies linearly with temperature (1 M) 1

2 F.3 Physics Mid Year Examination page 2 Length/cm (iii) Your teacher said that the measurement was not accurate and it was necessary to obtain more data for calibration. The following table shows the data obtained. Length/cm Temperature/ o C (1) Suggest one type of thermometer that can be used to measure the temperatures for calibration. (1 mark) Mercury in glass thermometer etc. (1 M) (2) Plot a graph for the calibration. (4 marks) Temperature/ 0 C (3) Calculate the % error in measuring the temperature (i.e. when the length of the liquid column reads 12 cm) using the method in (c)(i). Show your steps clearly. The true temperature = C (range 33-36) (1 M) Thus % error = /34.5 (1 M for correct step) = -7.25% (1 M for numerical answer) (4) What is the range of temperature in which the assumption in (c)(ii) holds? Explain briefly. Between C (1 M) Because the line is straight (1 M) 2

3 F.3 Physics Mid Year Examination page 3 2. An experiment is performed to measure the specific heat capacity of liquid P. Figure 2 shows the apparatus provided. on Liquid P a glass beaker a polystyrene cup thermometer instrument X stopwatch stirrer d.c. power supply immersion heater Figure 2 in out (a) Name instrument X and state its function. Joulemeter ( 1 M) To measure the amount of energy delivered (1 M) (b) Draw a set up diagram for the experiment. (Note: you may not use all the apparatus.) Heater immersed in liquid fully and the polystyrene cup is choosen. (1 M) Heater connected to joulemeter then d.c. power supply correctly (1 M) Stirrer and thermometer are used but no stop watch is used (1 M) (c) Suggest one way to improve the set up in (b). Explain briefly. Add a lid (1 M) To reduce heat lost to surroundings (1 M) and improve the accuracy of result (d) The experiment was performed using liquid P and the results were: Mass of liquid P used = 400 g Temperature of P/ o C Reading of X/ J Initial Final Calculate the specific heat capacity of liquid P using the given data = (0.4)(c)(40-25) (1 M for either one term correct) c = 2.30 x 10 3 J kg -10 C -1 (1 M) (e) State how the result will be affected if each of the following measures is taken. Circle your answers. (i) The heater is not fully immersed in the liquid. The measured specific heat capacity is expected to be larger than/the same as/smaller than the true value. (ii) A much longer time is taken for the experiment. (1 M) The measured specific heat capacity is expected to be larger than/the same as/smaller than the true value. (1 M) 3

4 F.3 Physics Mid Year Examination page 4 3. An electric heater is installed in the bathroom of a house. The heater can increase the temperature of tap water from 15 o C to 45 o C and it provides 3 kg of hot water every minute. (Given: Specific heat capacity of water = J kg -1 o C -1 ) (a) Find the energy supplied to heat the water every minute. E = mc T = (3)(4200)(45 15) = 378 kj (b) Hence, find the power output of the heater in watts. Power = E/t = ( )/(60) = 6300 W 4. Figure 4 shows a highly thermal insulated system used to find the energy stored in different kinds of foods. It basically consists of a metal container filled with water, underneath which food is combusted (burnt). The energy released by the combusting food is used to heat up the water in the container, and the temperature change of the water is measured by a thermometer. thermometer container water Combusting food Highly insulated capsule (a) Explain why it is needed to have (4 marks) (i) the capsule `highly insulated. prevent heat lost to surrounding, Figure 4 (ii) so as to improve accuracy. the water container made of metal. To facilitate heat transfer to water (metal is a good conductor of heat) to ensure heat is uniform around water. (b) When a specimen of 2 g of peanuts is combusted completely, the data below are obtained. Mass of water in container = 1 kg Change of temperature of water = 5 o C Specific heat capacity of water = J kg -1 o C -1 Heat capacity of container = J o C -1 Using the above data, find the energy stored in 1 g of peanuts. (4 marks) Heat absorbed by water and container = (4200)(1)(5) + (1500)(5) 1+ = J energy stored in 1 g of peanut = J 4

5 F.3 Physics Mid Year Examination page 5 (c) In reality, a small amount of heat is lost to the surrounding. State and explain if the following modifications, taken separately, could improve the accuracy of the experiment. (4 marks) (i) Replacing the container with another one of higher specific heat capacity. (ii) Using more water in the container. (i) is correct, as the specific heat capacity is high, temperature rise of container is less, less heat loss to surrounding. (ii) is also correct, as more water results in higher heat capacity, temperature rise is again less and thus less heat loss to surrounding g of water was heated at a constant power, the variation of its temperature (in o C) with time (in minute) in the first 40 minutes was plotted. However, a student accidentally poured some water onto the graph paper and hence the middle portion of the graph is missing as shown in Figure 5. The portions of the graph shown in the figure are both straight lines. (Given specific heat capacity of water = J kg -1 o C -1 ) Temperature / o C Figure Time / minute *(a) Explain the shapes of the two portions of the graph. (5 marks) In the first 3 minutes, the temperature rises linearly with time since it is heated at constant rate with little heat loss at low temperature. In the last 3 minutes, the temperature remains constant as rate of heat loss is large enough at that high temperature so that the rate of power supply equals the rate of heat loss. *com (b) By considering the first 3 minutes, estimate the power of the heater. E = (0.6)(4200)(30 10) = J P = E/t = / 3(60) = 280 W 5

6 F.3 Physics Mid Year Examination page 6 (c) From your answer in (b), find the total heat loss to the surroundings in the first 40 minutes. Heat loss = (280)(40)(60) (0.6)(4200)(50 10) 2M = J (d) Add the missing part of the curve on Figure 5. (1 mark) g of noodles is cooked by boiling water, and then cooled under running water at 20 o C. The running rate of the water is 20 g s -1. (Given the specific heat capacity of water = J kg -1 o C -1.) (a) The specific heat capacity of cooked noodles is larger than that of uncooked ones. Explain briefly. Cooked noodles contain more water than dry noodles, while water has a high specific heat capacity. (b) What is the advantage of cooling the noodles by running water? (c) Shorter time needed, etc. (1 mark) Given the specific heat capacity of the cooked noodles is J kg -1 o C -1, find the time needed for the temperature of the noodles to drop to 30 o C. (Take the initial temperature of the cooked noodles as that of the boiling water.) (4 marks) (0.5)(100 30)(2 200) = m(4200)(30 20) m = kg t = 1.833(1000)/20 = 91.7 s = 1.53 minutes (d) State two assumptions in your calculation in (c). 1. no heat loss to surrounding. (e) 2. temperature of noodles are uniform, etc. Should you expect the practical time needed for cooling the noodles to be longer or shorter than that found in (c)? Give a reason for your answer. shorter, if heat is loss to surrounding. OR longer, if not all water can exchange heat with noodles M End of Section A 6

7 F.3 Physics Mid Year Examination page 7 SECTION B (30 marks) Write your answers in the spaces provided on the last page of this question paper. 1. A thermometer reads 50 o C and the measurement has a maximum error of +10%. The possible value of the true temperature is: A o C B o C C o C D o C 2. When a solid is melting, which of the following statements is/are correct? (1) Energy is absorbed. (2) The average kinetic energy of the molecules is increasing. (3) The total internal energy of all molecules remains the same. A. (1) only B. (3) only C. (1) and (2) only D. (2) and (3) only 3. Which of the following thermometers is the most suitable in measuring the temperature of a furnace? A. thermistor B. resistance thermometer C. mercury in glass thermometer D. clinical thermometer 4. Which of the following actions is/are useful in improving the accuracy of the experiment to measure the specific heat capacity of metal? (1) Repeat the experiment a few times and take the average of values obtained. (2) Cool the metal block to a few degrees lower than the room temperature before the experiment. (3) Surround the metal block with a thick layer of paper. A. (1) only B. (1) and (2) only C. (2) and (3) only D. (1), (2) and (3) 5. Which of the following phenomena is NOT related to the high specific heat capacity of water? A. Water is used to fill the cooling system of a car engine. B. Sweating helps to cool our body. C. Coastal areas usually have warmer summers than inland areas. D. Water is used to put out a fire. 6. The graph shows the variation of temperature of two substances P and Q when they are separately heated by two heaters X and Y. Which of the following deductions is/are possible? (1) The mass of P and Q are the same while the power of X is higher than that of Y. (2) The power of X and Y are the same while the mass of P is larger than that of Q. (3) The power of X and Y are the same while P has a lower heat capacity than Q. A. (1) and (2) only B. (2) and (3) only C. (1) and (3) only D. (1), (2) and (3) 0 Temperature P Q time 7. If there is no heat flow between two bodies when they are in contact, which of the following quantities of the two bodies must be the same? A. average internal kinetic energy of the molecules B. total internal kinetic energy of the molecules C. specific heat capacity D. heat capacity 7

8 F.3 Physics Mid Year Examination page 8 8. A student uses an electric kettle to heat 0.5 kg of water at 40 o C. The water boils in 4 minutes. Estimate the output power of the kettle. The specific heat capacity of water is 4200 J kg -1 o C -1. A W B. 350 W C. 525 W D. 315 kw 9. The specific heat capacities of two different kinds of liquids X and Y are 800 J kg -1 o C -1 and 2400 J kg -1 o C -1 respectively. The energy needed to raise the temperature of a mixture of 3 kg X and 1 kg Y by 1 o C is A J. B J. C J. D J. Question 10 and 11 A metal block is first immersed in boiling water for some time. The block is then transferred to a cup of cold water. After a while, the temperature of the water is measured. Hot metal block Step 1: Step 2: Beaker Metal block Boiling water Heater Cup Thermometer Cold water The result of the experiment is as follows: Mass of metal block = 0.8 kg Mass of water in the cup = 0.3 kg Initial temperature of water in the cup = 23 o C Specific heat capacity of the metal = 381 J kg -1 o C -1 Specific heat capacity of water = 4200 J kg -1 o C Find the final temperature, in o C, of the metal and the water in the cup. A. 23 B. 38 C. 62 D The result obtained in the experiment is found to be higher than that found in Q.10. Which of the following is a possible reason? A. Some energy is gained from the surroundings to the cold water. B. Some energy is lost to the surroundings when the metal block is transferred to the cold water. C. Some energy is absorbed by the cup. D. The metal block is transferred to the cold water before it has reached thermal equilibrium with the boiling water. 8

9 F.3 Physics Mid Year Examination page 9 Directions: Each question below (questions 12 15) consists of two statements. Decide whether each of the two statements is true or false. If both are true, then decide whether or not the second statement is a correct explanation of the first statement. Then select one option A, B, C or D according to the following table: 1 st statement 2 nd statement A. True True 2 nd statement is a correct explanation of the 1 st statement. B. True True 2 nd statement is not a correct explanation of the 1 st statement. C. True False D. False True 1 st statement 2 nd statement 12. Water is not suitable to be used in a Water is colourless. liquid-in-glass thermometer. 13. Heat is transferred from an object of higher temperature to that of lower temperature. 14. When heat exchange occurs between two objects without heat loss, the temperature changes of the objects are always the same. 15. Objects at higher temperature shows a higher rate of heat loss to the surroundings. An object possesses more internal energy when it is at a higher temperature. The temperature change of a body is directly proportional to the heat transferred into or from that body. The specific heat capacity of an object increases at higher temperatures. Answers for Section B End of Section B 1. C 6. C 11. A 2. A 7. A 12. B 3. B 8. C 13. B 4. B 9. C 14. D 5. B 10. B 15. C End of Examination 9